The present invention relates to a zeolite with structure type MTT in the form of crystals and crystal aggregates with a specific ganulometry and to a catalyst comprising such a zeolite. The present invention also relates to the use of said catalyst in the principal transformation processes used in refining, in particular in straight chain paraffin isomerisation processes intended to improve the quality of such paraffin fractions for upgrading them, in particular into vehicle fuels. To this end, the isomerisation reaction can transform straight chain paraffinic hydrocarbons (normal paraffins, nP) with low octane numbers into branched hydrocarbons from the same family (isoparaffins, iP) with much higher octane numbers.
The isomerisation reaction leads to secondary reactions, coking and cracking, and conventional catalysts used to carry out such a reaction, namely bifunctional catalysts generally comprising an acid function and a hydrodehydrogenating function, lack selectivity for isoparaffins to the advantage of the secondary reactions cited above that constitute substantial losses for the desired reaction.
Further, in order to minimize secondary reactions, the Applicant has studied the synthesis of novel bifunctional zeolitic catalysts that are more active and more selective for converting straight chain parafffins than known catalysts.
In particular, the physical characteristics inherent in the catalyst have been studied and it has been discovered that the crystal size, and more precisely the size of the aggregates formed by the crystals of the zeolite comprised in said catalyst, have a major influence on paraffin hydrocarbon transformation catalytic performance in terms of activity and selectivity.
Zeolites with a specific granulometry have already been described in the prior art. As an example, European patent application EP-A2-0 323 893 describes an L zeolite that can be used to convert hydrocarbon feeds, which possesses aggregates of crystals with a length in the range 0.50 to 1.50 xcexcm and with a diameter in the range 0.2 to 0.6 xcexcm. That patent discloses that the aggregate size depends on the alkalinity of the reaction mixture during preparation of the zeolite. In their International patent application WO-A-93/25476, Verduijn et al. describe a ZSM-5 zeolite comprising aggregates in the form of needles with a maximum average length of 10 xcexcm, obtained by controlling parameters such as the crystallization temperature or alkalinity of the reaction mixture. Other authors (WO-A-93/08125) also suggest controlling the crystal and aggregate size of a molecular sieve formed from an MFI, MEL or xcex2 type zeolite by means of the crystallisation temperature. It should be noted that these prior art patent application never mention the specific size of the crystals and aggregates in combination with particular catalytic properties.
The Applicant has made significant progress by developing a catalyst containing a zeolite with structure type MTT, in particular ZSM-23 zeolite, having well defined crystal and crystal aggregate sizes, so as t reduce by a maximum the formation of undesirable cracking products and coke during isomerisation reactions.
ZSM-23 zeolite with structure type MTT, which has already been described in the prior art, has a unidimensional microporous framework, with a pore diameter of 4.5xc3x975.2 xc3x85 (1 xc3x85xc3x971 Angstrom=1xc3x9710xe2x88x9210 m) (xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d, W. M. Meier and D. H. Olson, 4th edition, 1996) Further, A. C. Rohmann et al (Zeolite, 5, 352, 1985).J. L. Schenker et al (private communication, 1992) and B. Marler et al (J. Appl. Cryst. 26, 636, 1993) have stated that the crystalline lattice has orthorhombic symmetry (Pmn21, a=21.5 xc3x85, b=11.1 xc3x85, c=5.0 xc3x85)with channels parallel to axis c, delimited by rings of 10 tetrahedra. The synthesis mode and physico-chemical characteristics of ZSM-23 zeolite have been described in a variety of patents which differ in the nature of the organic template used. That zeolite can be synthesised using pyrrolidine U.S. Pat. No. 4,076,842), diisopropanolamine (British patent GB-A-2 190 910), quaternary ammonium compounds such as heptamethonium bromide (U.S. Pat. No. 5 405 596) octamethonium bromide (GB-A-2 202 838) dodecamethonium bromide (U.S. Pat. No. 5 405 596) and quaternary triammonium compounds (U.S. Pat. No. 5,332,566) The mode of synthesis compries mixing an oxide, generally a silicon oxide, and an oxide, generally an aluminium oxide, in the presence of the template.
Other zeolite structure type MTT and differ from ZSM-23 zeolite in the mode of preparation, in particular in the organic template used. These are EU-13 zeolite (European patent EP-A-0 108 486), using a quaternary methylated ammomium or phosphonium salt, ISI-4 zeolite (EP-A- 0 102 497) using ethylene glycol or a monoethanolamine, SSZ-32 zeolite (U.S. Pat. No. 4 483 835) using imidazole derivatives or KZ-1 zeolite using a variety of amines (L. M. Parker et al, Zeolite, 3,8, 1988)
The present invention concerns a zeolite with structure type MTT comprising MTT zeolite crystals with a size of less than 5 xcexcm, at least a portion of the MTT zeolite crystals being in the form of MTT zeolite crystal aggregates, said aggregates having a granulometry such that the dimension Dv,90 is in the range 40 nm to 100 xcexcm. More particularly, the invention concerns ZSM-23 zeolite with structure type MTT, and its use as an acidic solid in the composition of a catalyst for isomerising light straight chain paraffins.
MTT zeolite, for example ZSM-23 zeolite as defined in the present invention, used as a catalyst in association with at least one binder, at least one metal selected from elements from group VIII of the periodic table, has improved catalytic performances for hydrocarbon transformations in terms of activity and selectivity, such as isomerising light paraffinic hydrocarbons containing 5 to 10 carbon atoms per molecule.
The zeolite with structure type MTT of the invention comprises crystals of MTT zeolite wherein at least a portion is in the form of aggregates of MTT zeolite. The zeolite with structure type MTT of the invention can be ZSM-23 zeolite, EU-13 zeolite, ISI-14 zeolite or KZ-1 zeolite.
Throughout the remaining text, the term xe2x80x9caggregatexe2x80x9d as used in the present invention means an ensemble formed from at least two zeolite crystals having at least one mutual point of contact. The granulometry of the crystal aggregates is represented by the dimension Dv,X, defined as the diameter of the equivalent sphere such that the size of X% by volume of aggregates is less than said diameter.
The zeolite with structure type MTT of the invention is characterized in that the size of the MTT zeolite crystals is less than 5 xcexcm, limits included, at least a portion of the zeolite crystals being collected into aggregates, said crystal aggregates being characterized in that their granulometry is such that the dimension Dv,90 is in the range 40 nm to 100 xcexcm.
The zeolite with structure type MTT of the present invention generally has the following formula in its anhydrous form 0 to 20R2/nO; 0-10 T2O3: 100 XO2, where R represents a cation with valency n, X represents silica and/or germanium, T represents at least one element selected from aluminium, iron, gallium, boron, titanium, vanadium, zirconium, molybdenum, arsenic, antimony, chromium and manganese, the overall atomic ratio X/T being 5 or more and preferably more than 10.
The crystal size is determined by X ray diffraction and/or using an electron microscope. The aggregate size is determined by laser diffraction granulometry and/or by electron microscopy. The granulometry is measured by laser diffraction using the zeolite suspended in water. The size distribution of the aggregates, defined by volume, is calculated from light signals collected by detectors and applying the Fraunhofer theory. The granulometric characteristics of the zeolite with structure type MTT are advantageously obtained directly during synthesis of the zeolite. They can also be obtained using any method that can reduce the aggregate size after synthesis such as grinding, for example, or by adapting the forming conditions, for example the mixing conditions during extrusion.
Preferably, the MTT zeolite crystal size is less than 2 xcexcm. More preferably, it is less than 0.5 xcexcm and preferably less than 0.2 xcexcm. The granulometry of the crystal aggregates is such that the dimension Dv,90 is preferably in the range 40 nm to 80 xcexcm. In particular, Dv,90 is generally in the range 40 nm to 60 xcexcm, more advantageously Dv,90 is in the range 40 nm to 40 xcexcm.
The zeolite with structure type MTT of the invention is obtained, for example, using a preparation process comprising reacting an aqueous mixture with at least one source of at least one element X selected from silicon and germanium, at least one source of at least one element T selected from aluminium, iron, gallium, boron, titanium, vanadium, zirconium, molybdenum, arsenic, antimony, chromium and manganese, at least one nitrogen-containing organic compound Q acting as a template selected from alkylated polymethylene xcex1-xcfx89 diammonium salt derivatives and precursors of said derivatives. The mixture is reacted until the zeolite crystallises. The alkylated polymethylene xcex1-xcfx89 diammonium derivative, used in particular for synthesising ZSM-23 zeolite, is defined by the formula R1R2R3N""(CH2)nN+ R4R5R6, n being in the range 3 to 14 and R1 To R6, which may be identical or different, representing alkyl or hydroxyalkyl radicals containing 1 to 8 carbon atoms; up to five R1 to R1 radicals can be hydrogen
In a first mode for preparing the MTT zeolite of the invention, the zeolite crystal and crystal aggregate size is monitored during synthesis and depends on the set of crystallisation processes which are controlled by the synthesis parameters. More particularly, these parameters include supersaturation (reactant concentration), pH (alkalinity), ionic strength (adding salts), addition of solid seeds, the temperature profile and the characteristics of mixing and stirring.
Regarding controlling the crystal and crystal aggregate size by the rate of stirring applied in the reactor, it is advantageous to apply at least two substantially different successive stirring rates to the mixture. Preferably, stirring is carried out at a first stir rate then at a second stir rate which is substantially higher than the first stir rate. Thus, for example, the second stir rate is at least 5% higher than the first stir rate. More preferably, it is at least 20% higher than the first stir rate, more preferably at least 50% higher than the first stir rate. The time during which stirring is carried out by applying the first stir rate advantageously represents 90% of the total stirring time, more advantageously 95% of the total stirring time. Clearly, the type of reactor used to carry out the preparation of the zeolite of the invention defines the stirring rate to be applied to obtain the desired crystal and crystal aggregate size Further, the power dissipated by stirring and the volume of the reactor must be taken into consideration when selecting a suitable stirring rate.
In a particular implementation, the process for preparing the zeolite of the invention comprises introducing during synthesis seeds S of at least one zeolite which is identical to or different from the MTT zeolite. In general, the seed particle size must be calibrated to between 0.005 and 500 xcexcm, preferably in the range 0.01 to 40 xcexcm Thus seeds of at least one zeolite, for example with structure type LTA, LTL, FAU, MOR, MAZ, OFF, FER, ERI, BEA, MFI, MTW, EUO, LEV, TON and NES, IM-5 or a NU-85, NU-86, NU-88 zeolite or a zeolite with structure type MTT can be used. Preferably, the seeds used are constituted by seeds of at least one zeolite with structure type LTA, FAU, MOR, MFI or MTT. In a preferred implementation, the seeds are different from the MTT zeolite of the invention in their structure type or in the chemical composition of the crystalline framework.
The seeds S are introduced directly after their synthesis or after having undergone at least one of the steps selected from the following post synthesis steps: washing, drying, calcining and ion exchange. Seeds can be introduced at any point in the preparation of the MTT zeolite. The seeds can be introduced at the same time as the sources of the metal oxides based on elements X and T, the organic template, or its precursors. Preferably, the seeds are introduced after at least partial homogenisation of the aqueous mixture containing the metal oxide precursors based on elements X and T and the template or template precursor.
When preparing a material with structure type MTT that is, for example, ZSM-23 zeolite, the alkylated polymethylene xcex1-xcfx89 diammonium derivatives used as an organic template include alkylated derivatives of heptamethylene xcex1-xcfx89-diammonium, octamethylene xcex1xcfx89-diammonium, undecamethylene xcex1-xcfx89 diammonium, dodecamethylene xcex1-xcfx89-diammonium and especially methylated derivatives heptamethylene xcex1-xcfx89 diammonium, octamethylene xcex1-xcfx89 diammonium, undecamethylene xcex1-xcfx89 diammonium, dodecamethylene xcex1-xcfx89diammonium derivatives, more preferably still 1,7-N,N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2,-hexamethylheptamethylene xcex1-xcfx89 diammonium salts, 1,8-N,N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2,-hexamethyloctamethylene xcex1-xcfx89 diammonium salts, 1,11-N,N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2, -hexamethylundecamethylene xcex1-xcfx89 diammonium salts, 1,12-N,N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x2,-hexamethyldodecamethylene xcex1-xcfx89 diammonium salts, for example the halide, hydroxide, sulphate, silicate or aluminate. The alkylated polymethylene xcex1-xcfx89 diammonium derivatives can be obtained from precursors. Suitable precursors of the starting alkylated polymethylene xcex1-xcfx89 diammonium derivatives are in particular the related diamines together with alcohols, alkyl halides, alkanediols or the related alkane dihalides together with alkylamines, preferably trialkylamines. They can be used in situ or they can be preheated together in the reaction vessel, preferably in solution before adding the other reactants necessary for synthesis of the ZSM-23 zeolite.
In one particular implementation of the process for preparing the zeolite of the invention, independent or otherwise of the preceding implementations, it may be advantageous to add at least one alkali metal or ammonium salt P to the reaction medium. Examples which can be cited are strong acid radicals such as bromide, chloride, iodide, sulphate, phosphate or nitrate, or weak acid radicals such as organic acid radicals, for example citrate or acetate. This salt can accelerate crystallisation of zeolites with structure type MTT, for example ZSM-23 zeolite, from the reaction mixture and it can affect the size and shape of the crystals of said zeolites.
In accordance with the process for preparing the MTT zeolite of the invention, and more particularly ZSM-23 zeolite, the reaction mixture for synthesising said zeolite with structure type MTT advantageously has the following composition, expressed in the oxide form.
Preferably, the reaction mixture has the following composition, expressed in the oxide form:
and still more preferably, the reaction mixture has the following composition, expressed in the oxide form
where
X is silicon and/or germanium. Preferably, X is silicon.
T is at least one element selected from aluminium, iron, gallium, boron, titanium, vanadium, zirconium, molybdenum, arsenic, antimony, chromium and manganese Preferably, T is aluminium.
M represents an alkali metal or an ammonium ion. Preferably, M is sodium.
Q represents the organic template or precursors of said template;
S represents zeolite seeds expressed in their dried, calcined or exchanged form.
P represents the alkali metal or ammonium salt.
M and/or Q can be present in the form of hydroxides or salts of inorganic or organic acids provided that the OHxe2x88x92/XO2 criterion is satisfied.
The reaction mixture is normally caused to react under autogenous pressure, optionally adding a gas, for example nitrogen, at a temperature in the range 85xc2x0 C. to 250xc2x0 C. until zeolite crystals with structure type MTT form, which can take from 1 minute to several months depending on the reactant composition, the mode of heating and the mixture, the working temperature and the degree of stirring.
When the reaction is over, the solid phase is collected on a filter and washed and is then ready for subsequent operations such as drying, calcining and ion exchange.
The silicon source can be any one in normal use envisaged for zeolite synthesis, for example solid powdered silica, silicic acid, colloidal silica or dissolved silica Powdered silicas which can be used include precipitated silicas, in particular those obtained by precipitation from a solution of an alkali metal silicate such as ZEOSIL or TIXOSIL produced by Rhxc3x4ne-Poulenc, fumed silicas such as aerosils produced by Degussa and xe2x80x9cCabosilxe2x80x9d produced by Cabot, and silica gels. Colloidal silicas with a variety of granulometries can be used, such as those sold under trade marks xe2x80x9cLUDOXxe2x80x9d from Dupont, and xe2x80x9cSYTONxe2x80x9d from Monsanto.
Particular dissolved silicas which can be used are commercially available soluble glasses or silicates containing 0.5 to 6.0 and in particular 2.0 to 4.0 moles of SiO2 per mole of alkali metal oxide and silicates obtained by dissolving silica in an alkali metal hydroxide, a quaternary ammonium hydroxide or a mixture thereof.
More advantageously, the aluminium source is sodium aluminate, but it can also be aluminium, an aluminium salt, for example a chloride, nitrate or sulphate, an aluminium alcoholate or alumina itself which should preferably be in a hydrated or hydratable form, such as colloidal alumina, pseudoboehmite, boehmite, gamma alumina or an alpha or beta trihydrate. Mixtures of the sources cited above can be used. Combined sources of silicon and aluminium can also be used, such as amorphous silica-aluminas or certain clays.
To obtain the hydrogen form of the MTT zeolite of the invention, ion exchange can be carried out using an acid, in particular a strong mineral acid such as hydrochloric, sulphuric or nitric acid, or with an ammonium compound such as an ammonium salt, for example ammonium chloride, sulphate or nitrate. Ion exchange can be carried out by diluting once or more times with the ion exchange solution The zeolite can be calcined before or after ion exchange or between two ion exchange steps, preferably before ion exchange to eliminate all absorbed organic substances, provided that ion exchange is thereby facilitated.
As a general rule, the cation or cations of the zeolite with structure type MTT can be replaced by one or more cations of any metal, in particular those from groups IA, IB, IIA, IIB, IIIA and IIIB (including the rare earths), VIII (including the noble metals), also lead, tin and bismuth (the periodic table is that shown in the xe2x80x9cHandbook of Physics and Chemistryxe2x80x9d, 76th edition). Exchange is carried out using any water-soluble salt containing the appropriate cation.
In a further mode for preparing the zeolite of the invention, independent or otherwise of the preceding preparation modes, the zeolite of the invention is obtained by post-synthesis grinding. This grinding is carried out on a zeolite with an aggregate Dv,90 of more than 100 xcexcm. Any grinding technique which is known to the skilled person is suitable. This grinding can be carried out on an as synthesised zeolite, before or after calcining or after cation exchange, using a dry or wet procedure provided that grinding does not affect the crystallinity of the zeolite.
In a further mode for preparing the zeolite of the invention, which is independent or otherwise of the preceding modes, the crystal aggregate size of the MTT zeolite can be controlled by adjusting the forming conditions, for example the mixing conditions during extrusion.
The present invention also concerns the use of the zeolite prepared using the process of the present invention as an adsorbent to control pollution, as a molecular sieve for separation and as an acidic solid for catalysis in the fields of refining and petrochemistry.
As an example, when it is used as a catalyst, the MTT zeolite can be associated with an inorganic matrix which can be inert or catalytically active, and with an active phase The inorganic matrix can be present simply as a binder to keep the small particles of zeolite together in the different known forms of catalysts (extrudates, beads, powders, pellets), or it can be added as a diluent to impose a degree of conversion on a process which would otherwise proceed at too high a rate leading to clogging of the catalyst as a result of increased coke formation. Typical inorganic diluents are support materials for catalysts such as silica, the different forms of alumina and kaolinic clays, bentonites, montmorillonites, sepiolite, attapulgite, fuller""s earth, synthetic porous materials such as SiO2xe2x80x94Al2O3, SiO2xe2x80x94ZrO2, SiO2xe2x80x94ThO2, SiO2xe2x80x94BeO, SiO2xe2x80x94TiO2 or any combination of these compounds. Said inorganic matrix can be a mixture of different compounds, in particular of an inert phase and a catalytically active phase.
Said metallic phase with which the zeolite can be associated is introduced into the zeolite alone, the inorganic matrix alone or into the inorganic matrix-zeolite ensemble, by ion exchange or impregnation with cations or oxides selected from the following. Cu, Ag, Ga, Mg, Ca, Sr, Zn, Cd, B, Al, Sn, Pb, V, P, Sb, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Pt, Pd, Ru, Rh, Os, Ir and any other element from the periodic table.
The zeolite with structure type MTT of the invention can also be associated with at least one other zeolite and act as the principal active phase or as the additive.
In a further mode for preparing the zeolite of the invention, which may or may not be independent of the preceding implementations, the size of the crystal aggregates can be controlled by adapting the dispersion conditions for the different ingredients forming the apparatus of the catalyst prior to forming.
Catalytic compositions comprising the zeolite with structure type MTT can be applied to isomerisation, transalkylation and dismutation, alkylation and dealkylation, hydration and dehydration, oligomerisation and polymerisation, cyclisation, aromatisation, cracking and hydrocracking, reforming, hydrogenation and dehydrogenation, oxidation, halogenation, amine synthesis, hydrodesulphurisation and hydrodenitrogenation, catalytic elimination of oxides of nitrogen, ether formation and hydrocarbon conversion and to the synthesis of organic compounds in general. these reactions involving saturated and unsaturated aliphatic hydrocarbons, aromatic hydrocarbons, oxygen-containing organic compounds and organic compounds containing nitrogen and/or sulphur, also organic compounds containing other functional groups.
More particularly, the invention concerns the use of a zeolite with structure type MTT of the present invention as an acid component of a bifunctional zeolitic catalyst for isomerising straight chain paraffins containing 5 to 10 carbon atoms.
The catalyst of the invention can be used in any process for isomerisation (or hydroisomerisation) of C5-C10 paraffins, preferably C7-C10, more preferably C7-C9 and still more preferably C7-C8. The catalyst of the invention is particularly suitable for a process for preparing gasoline with a high octane number, combining catalytic isomerisation and separation. More particularly, it is suitable for the process described in French patent application FR-B-2 771 419, which comprises an isomerisation section and at least one section for separating dibranched and tribranched paraffins.
The formed catalyst of the present invention contains:
at least one zeolite with structure type MTT, for example ZSM-23 zeolite, characterized by a crystal and crystal aggregate granulometry such that the size of the MTT zeolite crystals is less than 5 xcexcm, preferably less than 2 xcexcm, more preferably less than 0.5 xcexcm, and still more preferably less than 0.2 xcexcm and the Dv,90 of the crystal aggregates is in the range 40 nm to 100 xcexcm, preferably in the range 40 nm to 80 xcexcm, more preferably in the range 40 nm to 60 xcexcm, and still more preferably in the range 40 nm to 40 xcexcm;
at least one hydrodehydrogenating function,
at least one matrix.
More precisely, the MTT zeolite based catalyst of the invention contains at least one matrix in an amount in the range 1% to 90%, preferably in the range 5% to 90%, more preferably in the range 10% to 85%. Examples of matrices used to form the catalyst are generally selected from alumina gel, alumina, magnesia, amorphous silica-alumina, and mixtures thereof. Techniques such as extrusion, pelletisation or bowl granulation can be employed to carry out the forming operation.
The hydrodehydrogenating function is ensured, for example, by at least one element from group VIII of the periodic table, preferably at least one noble element selected from the group formed by platinum and palladium. The quantity of noble group VIII metal with respect to the final catalyst is preferably less than 5%, more preferably less than 3% and still more preferably less than 1.5%. It is also possible to use at least one non noble metal from group VIII. The amount of said non noble metal from group VIII with respect to the finished catalyst is advantageously in the range 1% to 40% by weight, more advantageously in the range 10% to 30% Advantageously, the non noble metal is associated with a group VIB metal (preferably Mo or W).
Isomerisation (hydroisomerisation) is carried out in at least one reactor. The temperature is in the range 150xc2x0 C. to 350xc2x0 C., preferably in the range 200xc2x0 C. to 300xc2x0 C., and the partial pressure of hydrogen is in the range 0.1 to 7 MPa, preferably in the range 0.5 to 5 MPa The space velocity is in the range 0.2 to 10 liters of liquid hydrocarbons per liter of catalyst per hour, preferably in the range 0.5 to 5 liters of liquid hydrocarbons per liter of catalyst per hour. The hydrogen/feed mole ratio at the reactor inlet is such that the hydrogen/feed mole ratio in the effluent leaving the reactor is generally more than 0 01, preferably in the range 0 01 to 50, more preferably in the range 0.06 to 20.
The following examples illustrate the invention more precisely without in any way limiting its scope.